Labyrinth Lord basically takes the approach of the red box and blue box Moldvay rules and models them well.

The strengths of the book, in my opinion, are its effective organization and presentation of the the rules. It's easy to follow, spells and monsters are organized logically (alphabetical order by class). The charts and tables are easy to read, and pictures (while generally of only moderate quality) are placed well to help you find what you are looking for.

The rules themselves are pretty straightforward, like its inspirational source. They aren't complex and generally take the approach of "simpler is better." One chart for reactions. One chart for running away. One chart for attacking in melee, one for range and its modifiers. Etc. Nothing is more complex than it needs to be. Realism is sacrificed but it's not that kind of game.

Where the book is hampered is in some oddly written sentences and seeming inconsistencies. For example, the book says, under Cause Disease (pg 21): "The victim of this disease cannot be cured of damage from other spells . . .” This means the victim of a Cause Disease spell needs to fear only damage from other spells. Saying "by other spells" would have been more accurate. Another example of this is under Unicorns: "A lawful, virtuous maiden may only approach these shy creatures." Er, lawful, virtuous maidens can't do anything else except approach unicorns? Poor maidens! I think the author means "Only a lawful, virtuous maiden may approach these shy creatures." The book also repeatedly uses "affect" as a noun when it clearly should be "effect," which is a common confusion but easily fixed. Maybe I'm being too harsh but I've edited in the past and I teach English so grammar errors like that leap out at me.(Just a note - I’m planning on submitting this stuff I found to the publisher as errata, so maybe these criticisms will become inaccurate in future printings.)

Inconsistencies include things like listing damage bonuses for weapon-armed creatures. Gnolls do "2d4 or by weapon +1" but minotaurs do "1d6/1d6 or by weapon" and then their description notes they get a +2 damage with weapons. Why not "or by weapon +2"? Seems likely to be an oversight. Giant boars are only 2 HD and +1d4 damage tougher than boars, but are 30' long (3 squares in a 10' scale dungeon)? Venomous creatures list an onset time for their venom, but Pit Vipers don't. Sapient Swords (aka intelligent swords) are their own entry on the treasure table . . . and nowhere in the text does it mention the pluses to hit and damage for them. So they have intelligence, languages, special powers . . . and no pluses? That seems more like an oversight than a design decision. Energy drain is called out and defined at the beginning of the monster section, and then each energy draining undead re-defines it. It's these little errors and inconsistencies that detract from an otherwise excellent rules set. There are a lot of these scattered around, and it gets tiresome to parse them out or decide if what you are seeing (damage stat, energy drain rules, etc.) is really the whole story.

Is it well-supported?

No point in playing a current game that isn't well supported, right? Well, Labyrinth Lord (LL) has an official miniatures line, numerous modules (such as Stonehell Dungeon), and even an "Advanced" version for people who want to play an AD&D clone off this base and an "Original" version for people who want white box 1974 D&D style characters (elf, choose your class today, Fighting Man or Magic-User). You could easily use this with any other retro-clone based on Gygax/Arneson spawned games, and use any old D&D stuff without any conversions. And all of the rules are available free, and in hardcopy via Lulu.

Rating:Content: 4 out of 5. It is a complete game. It's easily a replacement for those out-of-print materials and it's all you need to run a game. Some missing or unclear content (spell descriptions, the Sapient Swords bit, how exactly energy drain works, etc.) lowers its score a bit, but it's close to a 5.Presentation: 3 out of 5. Easily readable, in general, but with a lot of oddly written sentences as mentioned above. Charts are clear and the text is attractive. The art is only so-so.

Overall: I'd play this game. I recommend it to others looking for an old-school game system that is recognizably D&D but also in print and easily available. The various nitpicks I have are because it's otherwise so good. I went ahead and purchased a copy of the Advanced Edition Companion because I was so impressed with this and with the free version I'd been perusing. Recommended.

I love this book, and not just for the content. I find it very aesthetically unified and pleasing as well. I know you mentioned that the art is only so-so, but personally I really dig Steve Zieser's art.

There are some oddities. For example, look at the armor list and compare it to B/X D&D. These divergences start to make sense though once one realizes that it is derived from the D20 SRD, even though it is supposed to play like B/X.

@Brendan: Art is pretty subjective, I'll give you that. As for the SRD stuff, yeah, I can see that too. I deliberately didn't do a line-by-line comparison with B/X because I figured it needs to stand on its own merits. But it's good - it keeps what I like about Moldvay Basic while bringing in some stuff I liked about AD&D and what I saw of 2nd edition and 3.0/3.5 D&D. With the AEC, it's the OSR clone that I would have made if I ever really did sit down and "upgrade" B/X to the game I'd want to play.